EFFECT OF WATER-VAPOR PRESSURE AND TEMPERATURE ON THE AMORPHOUS-TO-CRYSTALLINE HA CONVERSION DURING HEAT-TREATMENT OF HA COATINGS

X-ray diffraction was used to characterize the increment of crystallin ity of HA coatings after heat treatment. Coatings were heated over the temperature (T) interval of 300 degrees-460 degrees C with a partial water vapor pressure of 0.01 MPa and 0.001 MPa. Heat treatment also wa s done in air, as a contrast. It was found that the ratio (n) of the i ncrement of crystallinity to the crystallinity of the as-received HA c oatings was more significant for the coatings heated in atmosphere wit h water vapor than for those heated in air. This ratio also increased with water vapor pressure. The logarithm of the ratio increased linear ly with 1/T, indicating that the ratio is exponential to T. The reason might be that recrystallization of the amorphous phase is a diffusion controlled process; the nucleation rate and growth velocity of the cr ystallites are in proportion to the diffusion coefficient, which is ex ponential to the temperature (T). Incorporation of water vapor in the atmosphere during heat treatment may decrease the activation energy fo r diffusion, which helps raise the diffusion coefficient of the atoms. Thus recrystallization of the amorphous phase can he accelerated. (C) 1997 John Wiley & Sons, Inc.